Transmission electron microscopes (TEMs) allow observers to see extremely small features, on the order of nanometers. A TEM sample, however, must be sufficiently thin for electrons to pass through. TEM samples are typically between about 20 nm and 200 nm thick.
Several techniques are known for preparing TEM specimens. These techniques may involve either cleaving, chemical polishing, mechanical polishing, or broad beam low energy ion milling, or combining one or more of the above. The disadvantage to these techniques is that they are not site-specific and often require that the starting material be sectioned into smaller and smaller pieces, thereby destroying much of the original sample.
Other techniques generally referred to as “lift-out” techniques use focused ion beams to cut the sample from a substrate or bulk sample without destroying or damaging surrounding parts of the substrate. Such techniques are useful in analyzing the results of processes used in the fabrication of integrated circuits, as well as materials general to the physical or biological sciences. These techniques can be used to analyze samples in any orientation (e.g., either in cross-section or in plan view). Some techniques extract a sample sufficiently thin for use directly in a TEM; other techniques extract a “chuck” or large sample that requires additional thinning before observation. In addition, these “lift-out” specimens may also be directly analyzed by other analytical tools, other than TEM.
For example, U.S. Pat. No. 5,270,552 to Ohnishi et al. describes using a focused ion beam to extract a sample by first milling a rectangular hole next to an area of interest, and then directing the beam into the sidewall of the rectangle to cut a “floor” under an area of interest, the floor being nearly parallel to the substrate surface. The ion beam then partially cuts around the circumference of the area of interest, and a probe is attached to the sample to be extracted. After the probe is attached, the remainder of the circumference is cut with the ion beam, and the sample, including the area of interest, is removed by the probe to which it has been attached.
Another focused ion beam technique is described in U.S. Pat. No. 6,570,170 to Moore, which describes extracting out a sample by making a “U”-shaped cut and then cutting the sample at an angle from the missing side of the “U” to undercut and free the sample. After the sample is freed, a probe is attached to the sample and it is lifted out.
In one technique that creates a thin sample requiring minimal additional processing before TEM observation, a focused ion beam cuts two adjacent rectangles on a substrate, the remaining material between the two rectangles forming a thin vertical wafer that includes an area of interest. A U-shaped cut is made at an angle partially along the perimeter of the wafer, leaving the wafer hanging by a tab on either side at the top of wafer. A probe is connected to the sample, and then the tabs are cut using the focused ion beam, freeing the sample.
All of these methods are time consuming. As more and more TEM samples are required to monitor nanofabrication processes, a more efficient process is needed for sample extraction.